The present invention relates to a high current ion implanter. A particular application of the invention is in the production of ion implanters used more particularly in the manufacture of semiconductors.
It is known that in the field of material doping ion implantation offers numerous advantages compared with diffusion and epitaxy methods. It essentially comprises producing ions of groups III or V of the periodic system and directing them at the material to be treated. However, this method only gives good results if it proves possible to overcome the difficulty of obtaining a uniform implantation. Thus, in general ion implanters comprise means which make it possible to obtain this uniformity. They vary according to whether the implanter belongs to the category of medium or low current equipment or to the category of high current equipment. The boundary between these two categories depends on the mass and energy of the ions but is generally about 0.5 mA.
In the case of low current implanters two different scanning means makes it possible to obtain the desired uniformity. The first generally comprises two sets of plates to which are applied voltages which vary in sawtooth manner with a frequency of about 1 kHz. The plates of one set are perpendicular to the plates of the other set so that the ion beam can be deflected in two perpendicular directions. Thus, it comprises XY electrostatic scanning. The second means utilizes hybrid electrostatic-mechanical scanning, whereby the implantation surface is moved regularly and perpendicularly to the direction of electrostatic scanning.
To said two scanning means used in low or medium current implanters is often added an accelerating tube which serves to obtain a high ion energy. This tube can be associated with an electron suppressor located downstream and which prevents acceleration of secondary electrons through the accelerating tube and consequently limits the production of X-rays. It is also possible to have ion analyzing means which are able to select the desired ions. Said implanters can also comprise electrostatic refocussing means as well as beam deflecting means which serve to eliminate neutrals. Finally it can be provided with an electron beam repulsing electrode located in the vicinity of the target and which permits a correct current measurement.
For further information regarding these various means it is possible, for example, to refer to the report of J. H. Freeman published in Atom GB 05:8147, Scientific Administration AERE Harwell, Oxfordshire or the article by D. Aitken entitled "The Design Philosophy for a 200 kV Industrial High Current Ion Implanter" published in the Journal "Nuclear Instruments and Methods" 139, 1976, pp. 125-134, or to the work by G. Dearnalay entitled "Ion Implantation", Noth Holland Publishing Company, Amsterdam and London, 1973.
It is simple to check the implanted dose (which is the product of the ion current intensity times the irradiation period) in the case of the two scanning systems, i.e. the purely electrostatic scanning system or the hybrid electrostatic-mechanical scanning system. Due to the large number of passages of the beam on the samples, it is easy to set the end of irradiation which takes place at the end of a short minimum time. Moreover, this large number of passages makes it possible to eliminate the inevitable problems of intensity fluctuations of the beam (breakdowns or the like), the duration of the disturbances produced always being long in comparison with the smallest of the two scanning periods.
However, although these two scanning means are suitable for low or medium currents, they are considered to be unusable in high current implanters. According to the prior art, in the case of high current equipment electrostatic deflection means cannot be used due to the significance then assumed by the space charge phenomenon. Basically this phenomenon is as follows. In an intense ion beam the electrical repulsion forces are such that they tend to blow up the beam. However, this effect is compensated by the presence within the beam of low energy electrons resulting from collisions between ions and neutral atoms of the gas or the bombardment of solid parts of the installation. The ion beam can be completely neutralized by the presence of these electrons provided, however, that possible fluctuations in the intensity of the beam are of low amplitude or frequency.
If an attempt is made to modify the trajectory of a neutralized ion beam by electrostatic means, this simultaneously disturbs the electronic population present in the beam in such a way that the neutralization of the beam is affected. As the beam is no longer completely neutralized difficulties are encountered in its control.
For this reason the prior art has considered that electrostatic scanning systems are unsuitable for high current ion implanters. This was affirmed in particular by D. Aitken in the above-mentioned article and by W. C. Ko in an article entitled "High Current Electric Scanning Method for Ion Beam Writing" published in the Journal IBM Technical Disclosure Bulletin, Vol. 18, No. 6, Nov. 1975, pp. 1832-1835.
Therefore electrostatic and electrostatic-mechanical hybrid scanning systems had to be abandoned in the case of high current implanters and purely mechanical scanning systems had to be developed. They comprise, for example, maintaining fixed the impact zone of the beam and moving the samples in said zone. This movement can be obtained by one or more reels onto which is wound a belt conveyor for the samples. These reels are given a double movement, one comprising rotation about the axis and the other translation parallel to said axis. Combined these two movements give each sample a helical trajectory.
The rotation speed of the reels is for example 80 r.p.m. and the translation of the belt parallel to the axis of the reels is about 1 mm per revolution. If the belt conveyor is 200 mm wide and the impact zone has a height of 50 mm, the total amplitude of lateral displacement of the belt is 250 mm which requires 250 revolutions.
Such purely mechanical scanning systems have many disadvantages:
(1) Due to the low translation speed of the reel the number of revolutions performed by the same must be constituted by whole revolutions and must be predetermined as a function of the intensity of the beam.
(2) As in practice the intensity of the beam undergoes slow fluctuations the latter must be compensated either by making the intensity of the beam dependent on the displacements of the reel or conversely by making said displacements dependent on the intensity. It is obvious that such means are very complex and difficult to design and construct. They may even require the use of a data processor. They are described in the articles by Aitken and Freeman to which reference has been made hereinbefore.
Although the two above-mentioned disadvantages can be overcome, there are others which remain insurmountable:
(3) Since in the case of mechanical scanning the scanning cycles are at least 100 times longer than electrostatic scanning cycles, the condition indicated hereinbefore according to which the duration of the fluctuations must be less than the smallest of the scanning cycles can no longer be met. If it was desired to obviate this disadvantage it would be necessary to use very long implantation periods which would be unacceptable in practice and to reduce the intensity of the ion beam, thereby returning to the field of low current implanters.
(4) The target surface temperature and the partial rearrangement of the crystal lattice thereof are directly linked with the time during which the beam irradiates the same portion of the target. In the case of mechanical scanning this time is long and these problems become insurmountable in the case of certain manufacturing processes.
Magnetic scanning means have been proposed but they suffer from the difficulty of being unable to attain high scanning frequencies.
Thus, in the case of a high current ion implanter it is necessary to use electrostatic scanning in order to obtain a high scanning speed but this implies being able to provide means which are able to surmount the difficulties occurring during the non-neutralisation of the space charge.
With this aim it has been proposed to apply a magnetic field to the electrostatic deflection space, said field being perpendicular to the electrical scanning field and to the beam axis. This field obliges the electrons to oscillate along the axis of the ion beam with the electrical scanning field and to some extent makes it possible to retain the electrons in the beam between the deflecting plates. In this connection reference can be made to the article by W. C. Ko mentioned hereinbefore. However, this solution is onerous and complicated.